The present inventions apply generally to improved methods of wastewater treatment in treatment facilities employing an activated sludge process. More particularly, the inventions are directed to enhancing the secondary treatment process by selected reaeration, recirculation and nitrification techniques that result in improved volumetric loading, hydraulic capacity and nutrient removal over conventional activated sludge treatment techniques.
Wastewater generated by municipalities and industries water is commonly collected and routed to a treatment facility for the removal of a variety of physical, chemical and biological pollutants prior to being discharged into a receiving body of water. To effect the necessary treatment, many public and private treatment facilities employ both physical and biological treatment methods. Physical methodsxe2x80x94including screening, grinding and physical setting processesxe2x80x94are effective for the removal of larger and heavier solids in the wastewater. However, lighter, smaller solids and other soluble pollutants in the wastewater resist removal by physical methods. For these pollutants, biological treatment methods such as activated sludge and trickling filters are commonly employed.
In the activated sludge process, settled wastewater is introduced into a reactor where an aerobic microbial culture is maintained in suspension. The culture may include a variety of different strains of bacteria, protozoa and rotifers. The combination of this microbial culture and the wastewater is commonly referred to as mixed liquor. Aeration in the reactor creates an aerobic environment and maintains the mixed liquor in suspension. The microorganisms interact with the wastewater to create a biomass that is more amenable to physical settling. After a specified reaction period, mixed liquor is sent to a settling tank to separate and remove the accumulated solids. A portion of the settled solids is treated further and the remaining portion is returned to the reactor to maintain a specified microbial concentration in the mixed liquor.
A desirable microbial culture will decompose organic pollutants quickly and will form a floc to separate biosolids. Mean cell residence time (MCRT) is the average time the microbes are present to metabolize their food. For typical domestic wastewater, the mean cell residence time generally falls within the range of five to fifteen days. Within these limits the beneficial treatment qualities of the floc generally improve with increased residence time. There is a direct relationship between mean cell residence time and effluent waste concentration.
Effluent discharges from wastewater treatment works must meet certain water quality limitations for selected pollutant parameters which are specified in discharge permits issued in accordance with the National Pollution Discharge Elimination System (NPDES). In order to meet the permitted effluent limits, wastewater treatment facilities are designed for a specified peak hydraulic capacity and a peak volumetric pollutant loading. The specified peak capacity and loading fix the size of the treatment facility. Still, in areas of residential or industrial growth, increased water use leads to increased wastewater production that, in turn, leads to increased hydraulic loading. Industrial processes may also produce occasional xe2x80x9cshockxe2x80x9d loadings of pollutants that may overwhelm the pollutant removal capabilities of the existing biological treatment facilities.
With the conventional activated sludge process, the maximum recommended volumetric pollutant loading rate is 0.6 (kg BOD5 applied/m3-day). Some enhancements to the conventional activated sludge process can increase volumetric pollutant loading without compromising the quality of the effluent. The known processes include enhanced aeration techniques, contact stabilization and Kraus process systems. However, even with known enhancements, there is an upper limit for volumetric loading. For enhancement by the Kraus process, the upper limit is 1.6 (kg BOD5, applied/m3-day)1. Modified aeration may raise the limit to 2.4 (kg BOD5, applied/m3-day). Pure oxygen aeration systems may attain a volumetric loading of up to 3.3 (kg BOD5 applied/m3-day), but are rarely used due to high implementation and maintenance costs.
1 Metcalf and Eddy, Inc. 1979. Wastewater Engineering, Second Edition. McGraw Hill. 
When pollutant loading or hydraulic capacity limits are reached, treatment facilities face the risk of permit limit violations, the possibility of Federal or State enforcement action, and restrictions or prohibitions on domestic and industrial growth within the service area of the treatment works. Typically, wastewater treatment facilities undergo physical expansion to meet the needs of increased hydraulic loading. But, physical expansion is expensive and often requires additional land that may not be available adjacent to existing facilities.
Therefore, it is desirable to find a way to increase volumetric pollutant loading and hydraulic capacity without the need for physical plant expansion. A significant advantage of the present invention over prior art methods of enhanced activated sludge processes is that volumetric pollutant loading can be substantially increased with only minor modifications to existing physical facilities. In addition, it is also a feature and advantage of the present invention that the enhanced activated sludge process produces a sludge with improved settling characteristics. Improved settling characteristics allow increases in hydraulic loading without requiring an increase in the size of the physical elements of the activated sludge system. By the same token, new activated sludge treatment works can be constructed in smaller sizes and at lower costs than known systems. With the enhanced activated sludge process of the present invention, design parameters that reflect the increased hydraulic capacity and pollutant loading capability can be incorporated into the sizing of the required structural elements to reduce the construction cost of new treatment works.
Most operators of wastewater treatment facilities have little or no control over the quality of the influent coming to the treatment plant they operate. Variations in domestic and industrial water use necessarily give rise to hourly, daily and seasonal fluctuations in influent wastewater quality. In particular, certain industrial events can result in the discharge of a xe2x80x9cshockxe2x80x9d pollutant load into the wastewater collection system and ultimately into the treatment plant. Such shock loading can upset the balance of the microbial culture present in the activated sludge reactor with a resulting loss of wastewater treatment effectiveness. Shock loading events also raise the risk of violating NPDES permit limitations on effluent quality with corresponding potential penalties and fines. It is another advantage of the present invention that the enhanced activated sludge process offers improved resistance to upsets of the microbial culture. It is also a feature and an advantage of the present invention that operating conditions of the activated sludge reactor are maintained in a more uniform condition, more resistant to undesirable variation in influent water quality changes.
For the activated sludge process to function properly, certain nutrients must be available in adequate amounts. The principal nutrients are nitrogen and phosphorus. While these nutrients are necessary for wastewater treatment, they may cause problems for aquatic life in the receiving waters where the treated effluent is discharged. Accordingly, the concentration of these nutrients in wastewater effluent is often limited by the NPDES discharge permit of the treatment facility. In these situations wastewater treatment facilities must include nutrient removal as part of the overall treatment process. In some cases the influent is nutrient deficient, requiring both the addition of nutrients and their subsequent removal. It is known in the art that nutrients may be added into the activated sludge process by chemical addition or, if digesters are present at the treatment works, by the recycling of digester supernatant.
Nutrient removal may be accomplished by any one of many suspended growth and attached growth processes that are known in the art. However, these systems often require the construction and operation of additional reactors and clarifiers, adding substantially to the cost of wastewater treatment. Therefore, it is a feature of the present invention that the enhanced activated sludge process can provide effective nutrient removal without the construction of separate nutrient removal reactors.